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Properties of extruded polymers are strongly affected by molecular structure. For two different semi-crystalline polymers, low-density polyethylene (LDPE) and ultra-high molecular weight polyethylene (UHMWPE), this investigation measures the elastic modulus, plastic flow stress and strain-rate dependence of yield stress. Also, it examines the effect of molecular structure on post-necking tensile fracture. The static and dynamic material tests reveal that extruded UHMWPE has a somewhat larger yield stress and much larger strain to failure than LDPE. For both types of polyethylene, the strain at tensile failure decreases with increasing strain-rate. For strain-rates 0.001-3400 s´1, the yield stress variation is accurately represented by the Cowper-Symonds equation. These results indicate that, at high strain rates, UHMWPE is more energy absorbent than LDPE as a result of its long chain molecular structure with few branches.

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A design mode, in which a casing is filled with a charge initiated off‐centre (eccentric point initiation), is common in the field of explosion and structural protection. The fragment velocity distribution along the circumference of the casing is distinctly non‐uniform due to the difference in blast loading around the circumference of the casing. To quantify the fragment velocity distribution, two kinds of cylindrical rings with different structural parameters were adopted. Static explosive experiments with three eccentric coefficients (0, 0.5, 1.0) were conducted with pulsed X‐ray diagnostics. Using coefficients derived from experimental data and calculating the effects of both the eccentricity of initiation and angle around the circumference, an angle‐dependent ratio βθ of charge mass to casing mass has been derived as a mean to modify the fragment velocity formula of Gurney for this application. The derived formula was shown to correctly predict the fragment velocity distribution around the circumference of the cylindrical ring. The calculated velocity distributions show good agreement with the experimental results.

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Abstract:A hyper-viscoelastic constitutive model for polyurea by separating hyperelastic and viscoelastic behaviors has been put forward. Hyperelasticity represents the rate-independent responses at low strain rates, described by a three-parameter Mooney-Rivlin model and a third Ogden model. By fitting the quasi-static experimental data, the Ogden model is more appropriate to describe the hyperelastic behaviors for its better agreement at strain over 0.3. Meanwhile, viscoelasticity represents the rate-dependent responses at high strain rates, described by the Standard Linear Solids (SLS) model and the K-BKZ model. By fitting the experimental data of split Hopkinson pressure bar (SHPB), the SLS model is more appropriate to describe the viscoelastic behaviors at strain rates below 1600 s´1, but the K-BKZ model performs better at strain rates over 2100 s´1 because of the substantial increase of Young's modulus and the state of polyurea transforming from rubbery to glassy. The K-BKZ model is chosen to describe the viscoelastic behavior, for its low Root Mean Square Error (RMSE) at strain rates below 1600 s´1. From the discussion above, the hyper-viscoelastic constitutive model is chosen to be the combination of the Ogden model and the K-BKZ model.

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Impact experiments were conducted with a gas gun to investigate the impact-induced initiation thresholds of a polytetrafluoroethylene/aluminum (73.5 wt % and 26.5 wt %) composite. Targets of steel, aluminum, and lowdensity polyethylene materials and sample rods of four different lengths were used to decouple the effects of impact pressure and loading strain rate. By subjecting the samples to different loading conditions, it was shown that the impact-induced initiation of polytetrafluoroethylene/aluminum is decided by the impact pressure and the loading strain rate simultaneously. The impact pressure and strain-rate thresholds for initiation were arrested by the experiments. A 308 inclined steel target was used to produce a compression-shear configuration as a comparison with the normal impact experiments. The initiation was more likely to happen; it demonstrated a shear-induced initiation mechanism, and a lower initiation strain-rate threshold was observed under oblique impact. Based on the experimental results, two theoretical curves were proposed to predict the impact-induced initiation of polytetrafluoroethylene/aluminum under normal and 308 oblique impact.